1.1 Field of the Invention
The present invention relates generally to a new process for the interfacing of capillary electrophoresis to mass spectrometry via electrospray ionization. More particularly, it concerns a novel sheathless interface useful for capillary electrophoresis/electrospray ionization.
1.2 Description of Related Art
The combination of mass spectrometry (MS) with a high-efficiency separation technique such as capillary electrophoresis (CE) provides a powerful system for the analysis of complex biological mixtures (William 1996). Currently, electrospray ionization (ESI) serves as the most common interface between CE and MS. A particularly important feature of any CE-MS interface is the method used to establish and maintain the electrical connection at the CE capillary terminus. These connections serve to close the CE electrical circuit and to provide a connection for the ESI voltage. The methods used to maintain these connections can be divided in two general categories: sheath-flow interfaces and sheathless interfaces. Comprehensive reviews of current CE/ESI-MS interfaces have been described by Cai et al. (Cai and Henion 1995) and Severs et al. (Severs, et al. 1996).
The sheath-flow configuration has been the most widely used interface for CE/ESI-MS, especially in commercial instrumentation. Sheathed techniques include Smith and co-workers' coaxial system (Smith et al. 1988) and the liquid junction technique established by Lee and co-workers (Lee et al. 1989). Sheath-flow configurations have several advantages including simple fabrication, reliability, and case of implementation. However, these techniques also bear several disadvantages: (1) dilution of the analyte by the sheath liquid; (2) competition for available charge between the species present in the sheath-flow and the analyte in the ESI process (Gale and Smith 1993); and (3) effects on separation, solubility, or molecular conformation which vary according to sheath liquid composition (Thompson et al. 1993, Foret et al. 1994, Smith et al. 1991).
In an attempt to remedy these problems, Severs et al. (1996) recently developed a new CE/ESI-MS interface in which the terminus of the CE separation capillary and a short ESI emitter capillary are connected by polysulphone micro-dialysis tubing. The electrical connection is provided outside the membrane via an electrode in a small sheath liquid reservoir that serves to close the CE circuit and simultaneously establish the electrospray voltage.
The first successful sheathless coupling of CE with MS was reported by Olivares et al. (1987). In this initial work, the outlet of the CE capillary was terminated within a stainless steel capillary which functioned as both a CE electrode and the electrospray needle. In this configuration, however, there was a relatively large dead volume at the capillary terminus. The current version of the sheathless source uses a gold conductive coating at the CE
Wahl and Smith (1994) presented a somewhat similar CE-MS interfacing approach, however, the electrical connection to the end of separation capillary was made though a micro-hole 2 cm from the end of the CE column. The micro-hole was sealed by conductive gold epoxy, through which electrical contact was made. Results presented by the inventors' laboratory (Cao and Moini 1995) and others show that these types of designs increase sensitivity by avoiding dilution of the sample with the sheath liquid. However, the CE capillaries made with the conductive gold tip or with the conductive gold epoxy have a short lifetime due to degradation of the gold tip or conductive gold contact as a result of chemical, electrochemical, and/or electrical phenomena. This degradation is a gradual process which interrupts electrical contact to the CE terminus leading to changes in migration times, instability of the CE current, and an unstable electrospray (Wahl and Smith 1994).
Another approach to sheathless CE-MS coupling is that of Fang et al. (1994) in which electrical contact to the CE capillary terminus is made by inserting a platinum wire into the outlet of the CE column. While an advantage of this technique is its ruggedness, however, there are two disadvantages associated with this method: the potential of arcing between the wire at the CE outlet and the inlet of the mass spectrometer and a non-uniform spray at the CE capillary terminus.